Integrated coupler

ABSTRACT

A non-directional coupler including a semiconductor junction in series with a capacitor, the semiconductor junction being formed so that the threshold frequency short of which it behaves as a rectifier is smaller than the coupler&#39;s operating frequency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of couplers having thefunction of sampling part of the power present on a main line towards asecondary line for control and feedback. Couplers are generally used inclosed-loop gain-control systems to provide a real measurement of thepower.

2. Discussion of the Related Art

FIG. 1 very schematically shows an example of a conventional circuitusing a coupler. This example relates to the control of a poweramplifier 1 (PA) for amplifying a useful signal UTI for a transmitantenna 2. In this type of application, the transmit power is controlledwith a power reference PL. A coupler 3 is interposed between the outputof amplifier 1 and antenna 2 to extract data proportional to the poweractually transmitted. This data is exploited by a detector 4 (DET)providing a measured value MES to a comparator 5 with required power PL.Comparator 5 provides a control signal REG to amplifier 1.

Two large coupler categories are essentially known. A first categoryrelates to so-called distributed couplers, which are formed from coupledtransmission lines. A second category relates to couplers with localcomponents, based on capacitors and inductances.

Distributed couplers are directional, that is, they detect the directionof the measured power and are sensitive to dimensional variations of thelines. Such couplers are bulky due to the size of the lines to beformed, especially for radio frequency applications (from severalhundreds of MHz to a few GHz).

Couplers with local components are non-directional. They have theadvantage of having a large passband and of being more compact.

As illustrated in FIG. 1, a coupler is defined by four ports orterminals IN, DIR, CPLD, and ISO. Terminals IN and DIR are on the mainline while terminals CPLD and ISO define the coupled secondary line. InFIG. 1, terminal IN is on the side of power amplifier 1 while terminalDIR is on the side of antenna 2. Terminal CPLD is the terminal on whichis sampled the information proportional to the power in the main line.In a non-directional coupler, to which the present invention applies,terminals IN and DIR are one and the same and terminal ISO generallydoes not exist.

The main parameters of a non-directional coupler are:

the coupling factor (generally on the order of from 10 to 30 dB) whichcorresponds to the path loss between ports IN and CPLD (the other portbeing loaded with a standardized impedance, generally 50 ohms); and

the insertion loss in the desired passband which corresponds to the pathloss between ports IN and DIR (the other port being loaded with astandardized impedance, generally 50 ohms) and which is desired to be assmall as possible (smaller than 1 dB and preferably on the order of 0.5dB) to minimize the attenuation of the wanted signal due to the presenceof the coupler.

FIG. 2 shows the electric diagram of a conventional non-directionalcoupler with local components. Such a coupler is essentially formed ofthe association of two cells 31 and 32 respectively forming high-passand low-pass filters. Cell 31 comprises a capacitor C31 having a firstelectrode connected to transmit line 12 (confounded terminals IN andDIR) and having a second electrode connected, by an inductance L31, toground. The second electrode of capacitor C31 also constitutes an inputterminal of cell 32 formed of an inductance L32 connecting this secondelectrode to terminal CPLD, terminal CPLD being further grounded by acapacitor C32.

A disadvantage of passive couplers with local components such as thatillustrated in FIG. 2 is linked to the dispersions (on the order of 20%)of the inductive and capacitive components upon manufacturing thereof.Such dispersions are reflected on the coupler parameters, which aregiven for an operating frequency band.

Theoretically, it is also possible to form high-pass and low-passfilters based on resistive and capacitive elements to form a coupler.However, the required number of stages (filter order) results, inpractice, in a large size filter. Further, the dispersion problem isalso present for resistors.

Above all, such structures are, in practice, not integrable inhigh-frequency applications (over one hundred MHz) more specificallyaimed at by the present invention, due to the small required values,especially for capacitors (less than one picofarad).

SUMMARY OF THE INVENTION

The present invention aims at providing a novel integrable couplerarchitecture.

The present invention more specifically aims at providing anon-directional coupler, the parameters of which are free of thedispersion problems of conventional couplers with local components.

The present invention also aims at enabling easy and accurate setting ofthe values of the coupler components.

To achieve these and other objects, the present invention provides anon-directional coupler comprising a semiconductor junction in serieswith a capacitor, the semiconductor junction being formed so that thethreshold frequency short of which it behaves as a rectifier is smallerthan the coupler's operating frequency.

According to an embodiment of the present invention, said semiconductorjunction is formed in an epitaxial layer, the thickness of whichconditions the threshold frequency from which the junction no longer hasa rectifying function.

According to an embodiment of the present invention, said capacitor hasa value greater than 10 picofarads.

According to an embodiment of the present invention, the semiconductorjunction is sized to exhibit, at the coupler's operating frequency, aseries capacitance on the order of a few hundreds of femtofarads and aseries resistance on the order of a few tens of ohms.

The foregoing objects, features, and advantages of the present inventionwill be discussed in detail in the following non-limiting description ofspecific embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2, previously described, are intended to show the state ofthe art and the problem to solve;

FIG. 3 shows the diagram of an embodiment of a coupler according to thepresent invention;

FIG. 4 shows a coupler according to the present invention connected to adetector of a control loop of the type illustrated in FIG. 1;

FIG. 5 shows in a very simplified cross-section view, an example of theforming of a coupler in a silicon wafer according to the presentinvention.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numeralsin the different drawings. For clarity, only those components which arenecessary to the understanding of the present invention have been shownin the drawings and will be described hereafter. In particular, theexploitation that is made of the measurements performed by a coupleraccording to the present invention has not been described in detail, thepresent invention applying whatever the type of measurements performedand whatever the transmit line on which the coupler is connected.

A feature of the present invention is to form an integrated coupler inthe form of a semiconductor junction (PN) in series with a capacitor.

FIG. 3 shows an embodiment of a coupler 3 according to the presentinvention.

A PN junction 35 is connected by a first terminal (indifferently P or N)to transmit line 12 (confounded terminals IN and DIR) while its otherterminal is connected to a first electrode of a capacitor 36 having itsother electrode defining terminal CPLD of the coupler.

According to the present invention, PN junction 35 is used, not as arectifying element but, at the frequencies desired for the coupleroperation, to form a capacitor 351 in series with a resistor 352, bothof very small value. “Very small value” means a capacitance 351 of lessthan one picofarad and a resistance 352 of less than 100 ohms. The PNjunction is thus formed to avoid rectifying the signal at relativelyhigh operating frequencies (greater than some hundred MHz) chosen forthe coupler. According to a preferred example, it is formed with anintrinsic area (PIN diode), for example, in an epitaxial layer.

Capacitor 36 has the function of blocking the D.C. component. Its valueis sufficient to be neglected in the series association with capacitor351. Preferably, a value greater than 10 picofarads fulfils theserequirements. The function of capacitor 36 will be better understood inrelation with the description of FIG. 4 integrating the coupler in itsapplication.

FIG. 4 shows a coupler 3 according to the present invention shown in theform of a diode 35 in series with a capacitor 36, and having itsterminal CPLD connected to a detection circuit 4 providing a measurementsignal MES for a comparator. Circuit 4 is a conventional circuit, forexample, of the type illustrated in FIG. 1. It comprises a rectifyingelement 41 (for example, a diode) having its anode connected to terminalCPLD and having its cathode connected, via a capacitor 44, to ground.The cathode of element 41 is further connected, by a resistor 42, toterminal MES which is grounded by a capacitor 43. Capacitors 43 and 44form with resistor 42 a low-pass filter reducing the ripple of the D.C.signal sampled on terminal MES.

Detector 4 further comprises a temperature-compensated bias circuitsetting a level Vdc on the anode of diode 41. This circuit is formed oftwo resistors 45 and 46 in series between a terminal 47 of applicationof voltage Vdc and the anode of diode 41. The midpoint of this seriesconnection is grounded by a diode 48 in series with a resistor 49. Suchan assembly enables obtaining an exploitable level even for low powersof signals carried on main line 12 (smaller than 0 dBm). Without thisbiasing, diode 41 would be blocked for such levels. However, thepresence of this bias voltage requires using capacitor 36 to avoid acontinuous biasing of PN junction 35 of coupler 3, which would null outthe desired operation.

FIG. 5 illustrates an example of integration in a silicon substrate 7 ofa PN junction 35 of a coupler according to the present invention. Toobtain the desired absence of a rectifying effect, an epitaxial region71 is provided between a P⁺ doped region 72 and N⁺ doped substrate 7.This is an example, and the doping types may be inverted. A first(anode) contact 73 is taken on region 72 and a second (cathode) contact74 is taken on the N⁺ region that is, on substrate 7.

Other junction configurations may be envisaged/provided to respect theabsence of a rectifying effect at the desired operating frequencies. Forexample, the PN junction may be formed from a diode-assembled NPN-typebipolar transistor (connected base and collector).

The threshold frequency fs from which the PN junction no longerrectifies the signal is a function of the carrier transit time(designated as tt). This frequency is proportional to the inverse of thetransit time.

If the main line signal has a frequency greater than threshold frequencyfs, the voltage switches from a negative value to a positive value andconversely, with a periodicity smaller than the transit time. Theforward incursion is too steep to cause a current and the carrier iscarried off by the negative halfwave before recombining, and thus beforegenerating a rectified current. Under such circumstances, the PNjunction is considered as a capacitor in series with a resistor.

As a first approximation, it can be considered that the transit timeessentially is a function of the epitaxial layer thickness and of thecarrier diffusion coefficient. More specifically, time tt isproportional to W²/D, where W represents the thickness of the epitaxiallayer and D represents the carrier diffusion coefficient.

For a diffusion coefficient of carriers D on the order of 13 cm²/s,which is a usual value in present technologies, frequency fsapproximately is 1300/W²(fs in MHz and W in μm). Generally, in lightdopings used to form the diodes, diffusion coefficient D of the carrierscan be considered as being constant. Accordingly, the smaller thethickness of the epitaxial layer, the higher the frequency from whichthe PN junction does not have a rectifying behavior.

It should be noted that the doping level of the regions has but littleinfluence on the threshold frequency of the PN junction.

A specific example of application of the present invention relates tocouplers used in the field of mobile telephony (GSM and DCS). The valueof capacitor 351 is on the order of a few hundreds of femtofarads. Thisvalue resulting from the diode forming can be adjusted by setting,according to the desired response and taking into account possible straycapacitances, especially the epitaxy doping, the active surface, and theepitaxy thickness in case of a full depletion. Such a value iscompatible with frequencies on the order of one GHz.

Similarly, resistive component 352 is on the order of a few tens ofohms, which is compatible with the forming of a PN junction and, again,adjustable according to the desired characteristics, by setting theinterval between the P⁺ and N⁺ regions.

It should be noted that, in case of an integration with the detectioncircuit, the anode of the diode thus formed directly forms the terminalon which line 12 is connected, that is, the antenna and the output ofthe power amplifier.

An advantage of a PN junction to form the coupler is that, in the formof an active coupler, its parameters are controllable even for smallcapacitance and resistance values, with a much smaller dispersion(linked to the semiconductor technology). The “active” coupler thusbecomes integrable. It can then be integrated on a same chip as that ofthe detection circuit (4, FIG. 4).

The values to be given to resistance 352 and capacitance 351 by theconfiguration of junction 35 are determined by usual modeling andsimulation tools according to the desired and/or acceptable couplingfactor and insertion loss at the selected operating frequencies.

Of course, the present invention is likely to have various alterations,modifications, and improvements which will readily occur to thoseskilled in the art. In particular, the practical forming of a diodefulfilling the constraints given by the present invention to form acoupler is within the abilities of those skilled in the art based on thefunctional and sizing indications given hereabove. Further, the presentinvention applies for a lateral diode as well as for a diode made in avertical technology and whatever the type of formed diode (PN diode, PINdiode, etc.), provided that it is sufficiently slow with respect to thedesired operating frequencies.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

1. A non-directional coupler comprising a semiconductor junction inseries with a capacitor, the semiconductor junction being formed so thata threshold frequency of the semiconductor junction, short of which thesemiconductor junction behaves as a rectifier, is smaller than thecoupler's operating frequency, wherein said capacitor has a valuegreater than 10 picofarads.
 2. The coupler of claim 1, wherein saidsemiconductor junction is formed in an epitaxial layer, the thickness ofwhich conditions the threshold frequency from which the junction nolonger has a rectifying function.
 3. The coupler of claim 1, wherein thesemiconductor junction is sized to exhibit, at the coupler's operatingfrequency, a series capacitance on the order of a few hundreds offemtofarads and a series resistance on the order of a few tens of ohms.4. A non-directional coupler comprising a semiconductor junction inseries with a capacitor, the capacitor having a value greater than 10picofarads, the semiconductor junction having a threshold frequencybelow which the semiconductor junction behaves as a rectifier, whereinthe threshold frequency is less than the coupler's operating frequency.5. The coupler of claim 4, wherein the semiconductor junction isdisposed in an epitaxial layer, wherein a thickness of the epitaxiallayer determines the threshold frequency above which the semiconductorjunction no longer functions as a rectifier.
 6. The coupler of claim 4,wherein the semiconductor junction is sized to exhibit, at the coupler'soperating frequency, a series capacitance on the order of a few hundredsof femtofarads and a series resistance on the order of a few tens ofohms.